Device for checking linear dimensions of parts

ABSTRACT

Device comprising a support (21-23) with a seat (27, 27&#39;) housing a measuring cell (24, 24&#39;) having a feeler (6, 6&#39;) adapted to contact the part to be checked and with a strain gauge transducer (12-17) for detecting the feeler position. In order to obtain a device with very small overall dimensions and high accuracy and repeatability, the measuring cell (24, 24&#39;) comprises an integral element (1) having a support portion (2) fixed in the seat (27, 27&#39;), a free portion (4, 4&#39;) carrying the feeler (6, 6&#39;) and an intermediate resiliently deformable portion (3, 3&#39;), whereto there is fixed a strain gauge (13) of the transducer (12-17).

TECHNICAL FIELD

The invention relates to a device for checking linear dimensions ofparts, with a support structure defining a seat and a measuring cellfixed in a determined position in the seat, the measuring cell having afeeler adapted to touch the part to be checked, a flexible portion forpermitting displacements of the feeler and transducer means comprisingat least a strain gauge fixed to said flexible portion.

BACKGROUND ART

European patent application EP-A1-0129741 describes a device comprisinga flexible lamina secured in a seat of a support and carrying elementsof a strain gauge transducer. The lamina has a free end cooperating witha feeler rigidly coupled to a bushing movable within a guide body fixedto the seat. The seat is sealingly closed by a rigid cover, a sealingring arranged between the cover and the support and a resilientseal--having an opening for the feeler passage--coupled to the cover,the feeler and the guide body.

The known device can be manufactured with rather small overalldimensions, but the somewhat high number of constituent elementsprevents the reduction of the overall dimensions, involves complex andlong assembling operations and negatively affects the accuracy andrepeatability of the measurements, that also suffer from the presence ofa sliding contact between the feeler and the lamina.

Japanese patent application published under No. 52-35662 discloses astrain gauge device for checking the deformation of vessels subjected tohigh pressures. The device comprises a substantially C-shaped element,apparently obtained from a single piece, with a section of reducedthickness, so as to be resiliently deformable, whereto there are fixedstrain gauges. The device is not provided with a real frame or support,even if there is a ring adapted to cooperate with the arms of theelement for keeping them at a determined distance.

During the measurement, the ring is removed and the ends of the integralelement, that act as feelers, contact relevant points of the vessel tobe checked.

This device with two feelers is provided for very particularapplications, for which manual operations by a rather skilled operatorare necessary. Anyway, the accuracy and repeatability are inevitablypoor, due to the manner in which the device cooperates with the vessel.

German patent application DE-A-3003370 shows in FIG. 2 a device forchecking linear dimensions of parts comprising a support body thatdefines a seat and a measuring cell fixed within the seat. The measuringcell comprises an integral element having a support portion adapted tobe coupled to the support body, a movable end portion carrying a feelerand an intermediate flexible portion connecting the support portion withthe movable end portion. The intermediate flexible portion, that hassubstantially the shape of, and operates as a flat leaf spring, carriestransducer means constituted by strain gauges. A member defining aprotection cover terminates at one end with a coupling portion having ahole for a screw also passing through a hole of the support portion ofthe integral element, for locking the integral element to the supportbody.

German patent application DE-A-3207837 shows in FIG. 1 an electronicplug gauge for checking diameters of holes having an arm-set featuring asubstantially U-shaped integral member arranged within a substantiallycylindrical protection body. The integral member has two flexibleportions next to the base of the member and two legs or arms, each ofwhich is movable about a rotation axis defined by the correspondingflexible portion. Fixed at the ends of the movable arms are feelers forcontacting a diametrically opposite point of the hole. A positiontransducer comprises two elements respectively fixed to intermediateportions of the movable arms.

DISCLOSURE OF INVENTION

The invention is intended to overcome the drawbacks of the prior art, bypermitting the obtainment of a device with possible very smalldimensions, easy to assemble, very sturdy, and such as to guarantee highaccuracy and repeatability.

According to the invention, a device for checking linear dimensions ofparts, comprises a support body defining a seat and a measuring cellfixed in a determined position in the seat. The measuring cell includesa feeler adapted to touch the part to be checked, transducer meansproviding a signal depending on the feeler position, and a substantiallylinear integral element having a support portion adapted to be fixed inthe seat, a movable portion carrying said feeler and an intermediateportion with reduced thickness, that defines a flexible portion forpermitting displacements of the movable portion with respect to thesupport portion. The transducer means includes a strain gauge fixed tothe intermediate portion.

The effects and advantages offered by the invention are that it ispossible to carry out measurements of linear dimensions in zones withvery poor accessibility and to perform a plurality of simultaneousmeasurements in points very close to one another, by means of relevantmeasuring cells carried by one or more supports. The cheapness of thecells makes the maintenance simpler in the case of failures, byrendering advantageous the replacement of a whole cell, rather than itsrepair.

On the other hand, the small and compact structure of the measuringcells and their sturdiness render failures very unlikely.

Since the measuring cell and the relevant seat can be very small, themanufacturing of measuring devices featuring protection against foreignmatter is rendered easier.

The construction of the cell by an integral element, substantiallylinear, that defines a support portion, a fulcrum and a movable arm,enables the obtainment of very low thermal drift errors and thesimplification of the assembling and setting up operations of themeasuring devices.

By precision machining of the seat and of the cell, it is possible tomanufacture measuring devices not requiring mechanical zero settingoperations.

The structure of the measuring cells and the use of strain gaugetransducers, besides being intrinsically cheap, permit the use of powersupply, processing and indicating electronic units, they too cheap. Infact, it is known that the measuring signals of the strain gaugetransducers, even if have the limitation to be rather weak, offer theadvantages of involving low noise and of not requiring power supply orprocessing with alternate currents. Consequently, for example, theelectronic units do not necessitate the use of oscillators.

BRIEF DESCRIPTION OF DRAWINGS

The invention is now described in more detail with reference to theannexed drawings, given for exemplary purpose only, relating to ameasuring cell and to the application of two similar cells in acomparator plug gauge for checking internal diameters.

FIG. 1 is a longitudinal section of a measuring cell according to thebest mode for carrying out the invention;

FIG. 2 is a bottom view of the cell of FIG. 1;

FIG. 3 is a side view, from left to right, of the cell of FIG. 1;

FIG. 4 shows a partly sectional view of a part of an electronic pluggauge comprising two cells similar to those of FIGS. 1-3, apart fromsome variants;

FIG. 5 is a side view, from right to left, of the gauge of FIG. 4; and

FIG. 6 is a section of the gauge of FIGS. 4 and 5, along the planecorresponding to path VI-VI in FIG. 4.

BEST MODE FOR CARRYING OUT THE INVENTION

With reference to FIGS. 1 to 3, an integral member or element 1, made ofspecial steel for load cells, comprises a support portion 2 adapted tobe secured in a seat, an intermediate portion 3 having reduced thicknessand a portion 4 acting as a movable measuring arm and carrying, next toits free end, through a support block 5, a feeler 6 for contacting thepart of which it is desired to check linear dimensions, and an end stopand centering element 7 having spherical shape.

Support block 5 is fixed within a through hole of measuring arm 4 bymeans of a suitable adhesive, that is also used for securing feeler 6and element 7 to support block 5. Of course, the coupling can also beobtained by welding, or in another manner.

The support portion 2 comprises a heel like part, constituted by aprotruding part 8 having a central longitudinal recess 9 arrangedbetween two juts 10, 11, the bases of which are fixed in the seat.

Next to the upper part of support portion 2, the thickness of member 1decreases due to a transversal lightening, with circular arc outline,that defines portion 3, where the thickness is locally so reduced as topermit--as a consequence of the application to feeler 6, along its axis,of comparatively light forces--small rotational displacements of portion4, substantially about a transversal rotation axis defined by portion 3.Thus portion 3 can be assimilated to a rotation fulcrum for the movablearm corresponding to free portion 4.

Glued or welded to the lower face of integral element 1 is a straingauge strip 12, of commercial type, that has two strain gauges 13, 14,one of which, 13, in correspondence with portion 3 and the other, 14, incorrespondence with portion 4 (or, according to an obvious variant, withportion 2).

Strain gauges 13, 14, that can also be directly fixed to the integralelement, rather than to strip 12, are connected by three conductors 16,housed within a cable 17, in a bridge type circuit, on its turnconnected to a power supply, detection and indicating electrical unit.The bridge circuit and the electrical unit are not shown, because theyare commonly used in strain gauge devices.

As appears from FIGS. 2 and 3, a portion of cable 17 is housed withinrecess 9 of the heel shaped part 8.

The structure shown in FIGS. 1 to 3 permits manufacturing very smallmeasuring cells. For example, a cell prototype has been made includingan integral element 1 having the following features : 16 mm length;movable arm 4 with 3 mm width; 1.5 mm thickness in correspondence withportion 2 and heel 8; minimum thickness of 0.15 mm in correspondencewith portion 3; 0.95 mm thickness in correspondence with portion 4;distance of 9 mm between the rotation axis defined by portion 3 and thegeometric axis of feeler 6. This prototype of measuring cell is alsocharacterized by a measuring force (force between feeler 6 and the part)variable between -0.75 and +0.75 N, in a measurement range from -150 to+150 um.

Cells similar to that of FIGS. 1 to 3 can be used both in measuringdevices with a single feeler and in devices with a plurality of feelersfor checking one or more linear dimensions.

FIGS. 4 to 6 refer to a plug gauge comprising two measuring cells forchecking the diameter of a hole. The illustrated plug gauge is adaptedfor applications in transfer lines or, more generally, for automatictype checkings, but the invention offers advantages at least in partcommon for both manual and automatic plug gauges. Particularly importantamong these advantages are the possibility of manufacturing plug gaugeswith two or more pairs of feelers for checking small diameter holes (forexample 8 mm diameter) and at a high depth (for example of several tensof millimeters).

The gauge of FIGS. 4 to 6 comprises a support and protection structureor nosepiece, of which a first portion 21, an intermediate portion 22and an end portion 23 are visible in the figures. End portion 23 has aguide end and can be substantially solid, while the first portion 21 andintermediate portion 22 have several recesses.

In particular, first portion 21 can have a tubular shape, for reducingthe gauge weight and housing the cables 17, 17' of the two measuringcells 24, 24' fixed to intermediate portion 22.

Intermediate portion 22 has two cylindrical surface sections 25, 25'having a diameter slightly smaller than the maximum diameter of endportion 23. With reference to FIG. 6, the two sections 25, 25' areoppositely arranged along a direction transversal to the gaugelongitudinal axis, while two chamfered surfaces or shoulders 26, 26' areoppositely arranged along a transversal direction perpendicular to thepreceding one. In correspondence with surfaces 26, 26', intermediateportion 22 defines two seats 27, 27', housing measuring cells 24, 24',respectively. Since measuring cells 24, 24' are identical to each otherand are symmetrically arranged with respect to a geometric plane passingthrough the gauge longitudinal axis, only the arrangement of cell 24 isnow described. Moreover, as far as cell 24 is concerned, only thedifferences with respect to the cell of FIGS. 1 to 3 are described indetail.

Seat 27, as seen in its top view (FIG. 5), has a shape substantiallycorresponding to that of cell 24, with a slightly larger width forpermitting the insertion of the cell in the assembling phase and, duringthe operation, the movement of the movable portion or arm 4 about theportion 3 defining the rotation fulcrum.

Fixed within a hole 30 obtained in the longitudinal portion 31connecting sections 25, 25' is an element 32 that supports, also bymeans of a small plate 33, a ball 34 that can cooperate with movable arm4 in correspondence with the end of a cylindrical hole 35 housing ablock 36 supporting feeler 6.

Seat 27 is sealingly closed by a flat metal lamina 37, the edges ofwhich are glued or welded in a recess 38 obtained in the chamferedsurface 26 next to the recess defining seat 27. Part of lamina 37 is notshown in FIG. 5, for rendering visible recess 38. Lamina 37, that ismade of steel for springs and has a thickness of 0.2 mm, defines a hole39 for the passage of feeler 6.

The external edge of a resilient seal 40, made of rubber, is glued orvulcanized onto the upper face of lamina 37, next to the edge of hole39.

Seal 40 defines a central hole for the passage of feeler 6 and incorrespondence with this hole has an annular portion 41 that is glued orvulcanized onto block 36. The part of seal 40 between the external edgeand the annular portion has a thickness of 0.4 mm and is shaped so as topermit the measurement displacements of feeler 6 by applying an elasticreaction at least an order of magnitude smaller than the force due tothe resilient deformation of portion 3.

Lamina 37, due to its flat shape and the fixing along all its periphery,does not undergo substantial deformations, but its use and that of seal40 permit the sealing of seat 27 with a very small room expense.

Apart from feeler 6 and a portion of block 36, cell 34 (in particularthe relevant integral element) is completely housed within the roomdefined by seat 27, metal plate element 37 and resilient seal 40.Therefore cell 24 is very effectively protected against impacts andcontamination by foreign matter.

The measurement stroke of feeler 6 is limited due to the contact betweenball 34 and the surface of movable arm 4 that delimits hole 35.

Ball 34 and hole 35 are also important in the assembling phase foraccurately defining the position of cell 24 within seat 27. For thispurpose, after having applied quick setting adhesive onto supportportion 2 of cell 24 and/or onto the zone of seat 27 provided forhousing said portion 2, cell 24 is immediately inserted, possibly by atool, into seat 27. Then, by acting onto feeler 6, first referencemeans, constituted by the edge of hole 35, are displaced to steadilycooperate with second reference means, defined by ball 34. After a timesufficient to guarantee the adhesive setting, feeler 6 can be released.For cell 24', one can act in a similar way.

By this method, the obtainment of the alignment of the feelers of cells24, 24' along a diametral line is greatly simplified and basically onlydepends on the positioning accuracy of balls 34, 34', that can be kepthigh without particular technical and economic problems.

The exact alignment of cells 24, 24' along the longitudinal direction ofthe plug gauge is not essential and in any case this alignment can beobtained quite satisfactorily by taking as references, for example bymeans of the already mentioned tool, the side walls of seats 27, 27',that can be manufactured, without any difficulty, with the properaccuracy. The alignment can also be performed through the cooperationbetween suitable additional mechanical abutments obtained in seats 27,27' and portions 2, 2'.

Besides avoiding, through the described method, the need of adjustmentdevices to be used (when the gauge is already assembled) for aligningfeelers 6, 6' of cells 24, 24' along a diametral direction of the pluggauge (and thus of the part), the structure of measuring cells 24, 24'can be made very simple, without using devices for adjusting feelers 6,6' along the radial direction.

In fact, if the depth of seats 27, 27'--or, more exactly, the distanceof the surfaces of seats 27, 27' onto which portions 2 are fixed fromthe plug gauge axis--and the rest position of feelers 6, 6' with respectto portions 2, 2' are accurately obtained, the above describedassembling method guarantees the obtainment of the radial position offeelers 6, 6' with respect to the plug gauge axis with almost identicalaccuracy.

Therefore, after its assembling the plug gauge can be used without anyneed of performing mechanical zero setting operations.

We claim:
 1. Device for checking linear dimensions of parts, with asupport structure (21-23) defining a seat (27) and a measuring cell (24)fixed in a determined position to the seat (27), the measuring cell (24)comprising: a feeler (6) adapted to touch the part to be checked; asubstantially linear integral element (1) having a support portion (2)adapted to be fixed to the seat (27), a movable portion (4) carryingsaid feeler (6) and an intermediate portion (3) with reduced thickness,that defines a flexible portion for permitting displacements of thefeeler (6); and transducer means including at least a strain gauge (13)fixed to said flexible portion, characterized in that said flexibleportion is substantially adapted to define a rotation axis forpermitting rotational displacements of the movable portion (4) withrespect to the support portion, whereby said flexible portion isassimilable, substantially, to a rotation fulcrum for the movableportion.
 2. A device according to claim 1, wherein the transducer meanscomprise a cable (17) and wherein said support portion (2) comprises twoprotruding parts (10, 11) adapted to be fixed in the seat (27) and arecess (9) for the cable passage.
 3. A device according to claim 1,wherein said support portion (2) is glued in the seat (27).
 4. A deviceaccording to claim 1, wherein the support portion (2) has a thicknessgreater than the intermediate portion (3) and the movable portion (4).5. A device according to claim 1, wherein said integral element (1) ismade of steel.
 6. A device according to claim 1, wherein the transducermeans comprise a strip (12) carrying said strain gauge (13) and afurther strain gauge (14), the strip (12) being fixed to the integralelement (1).
 7. A device according to claim 1, wherein the transducermeans comprise a further strain gauge (14) fixed to the integral element(1) in correspondence with a portion (4) chosen between the movableportion (4) and the support portion (2).
 8. A device according to claim1, wherein said seat (27) has a predetermined depth and defines a baseadapted to provide an abutment for the rest position of the feeler (6),the feeler (6) having, in said rest position, a determined distance fromsaid base.
 9. A device according to claim 1, for checking internaldiameters, of parts, wherein the support structure (21-23) defines asecond seat (27'), the device including a second measuring cell (24')fixed to the second seat (27').
 10. A device for checking lineardimensions of parts, including a support structure (21-23) defining aseat (27) and a measuring cell (24) fixed in a determined position tothe seat (27), the measuring cell (24) comprising: a feeler (6) adaptedto touch the part to be checked; a substantially linear integral element(1) having a support portion (2) adapted to be fixed to the seat (27), amovable portion (4) carrying said feeler (6) and an intermediate portion(3) with reduced thickness, that defines a flexible portion forpermitting displacements of the feeler (6), the flexible portion beingsubstantially adapted to define a rotation axis for permittingrotational displacements of the movable portion (4) with respect to thesupport portion; and transducer means including at least a strain gauge(13) fixed to said flexible portion; and wherein first and secondreference means (7; 34, 35) are associated with the seat (27) and saidmovable portion (4), respectively, and are arranged in correspondencewith said feeler (6) for obtaining a positioning of the feeler (6) withrespect to the seat (27) and permitting the subsequent fixing of thesupport portion (2) to the seat (27).
 11. A device according to claim10, wherein the first and second reference means comprise a sphericalsurface (34) and a cylindrical hole (35).
 12. A device according toclaim 10, wherein the first and second reference means (34, 35) areadapted to cooperate with each other, during the device operation, forlimiting the displacements of said movable portion (4).
 13. A device forchecking linear dimensions of parts, including a support structure(21-23) defining a seat (27) and a measuring cell (24) fixed in adetermined position to the seat (27), the measuring cell (24)comprising: a feeler (6) adapted to touch the part to be checked; asubstantially linear integral element (1) having a support portion (2)adapted to be fixed to the seat (27), a movable portion (4) carryingsaid feeler (6) and an intermediate portion (3) with reduced thickness,that defines a flexible portion for permitting displacements of thefeeler (6), the flexible portion being substantially adapted to define arotation axis for permitting rotational displacements of the movableportion (4) with respect to the support portion; and transducer meansincluding at least a strain gauge (13) fixed to said flexible portion;and wherein said seat (27) is sealingly closed by a metal plate element(37) fixed to the support structure (21-23) and a resilient seal (40)fixed to the metal plate element (37) and to the feeler (6), the metalplate element (37) and the resilient seal (40) defining relevant holes(39) for the passage of the feeler therethrough.
 14. A device accordingto claim 13, wherein said integral element (1) is completely housedwithin the room defined by said seat (27), metal plate element (37) andresilient seal (40).